The present invention relates generally to lasercom acquisition and tracking systems and more particularly to an apparatus an method for determining a pointing uncertainty of a satellite communications systems.
Space based communications systems are mounted on spacecraft and provide communications between two terminals on separate spacecraft. For high data rate transfers, some spacecraft systems use Laser signals for communications, herein referred to as Lasercom. Lasercom communications beams are typically very narrow and as such, must be precisely directed towards the opposing terminal to communicate. Factors such as uncertainty in attitude and position of the terminals as well as the uncertainty of the attitude and position of each spacecraft itself can result in the Lasercom signal being mispointed. To solve this problem, each satellite which has a Lasercom system typically provides hardware and software which enables one satellite to acquire the opposing terminal.
One method used to acquire the opposing satellite in a lasercom system is to provide a separate mechanism on one spacecraft which is used for acquiring the opposing spacecraft. To do so, a beacon having a beamwidth which is as broad as the pointing uncertainty is mounted on each spacecraft. In addition, a sensor having a field-of-view which encompasses thehhe field-of-view increases. As such, the beamwidth of the beeeacon and the sensor must be increased to cover the wider field-of-view. This is undesirable since a wider beamwidth beacon requires additional power which is a scarce commodity on a spacecraft.
Another method used to acquired an opposing satellite in a lasercom system is to mount a beacon having a narrow beamwidth and a sensor having a wide field-of-view on one spacecraft. The narrow beacon is scanned in the vicinity of the opposing satellite and eventually, the opposing terminal is acquired. This is referred to as the narrow beacon approach. A downfall of this approach is that the field-of-view which must be scanned increases with an increase in the uncertainty. As such, as the uncertainty increases, the field-of-view which must be scanned increases in an amount proportional to the uncertainty squared since the field-of-view which must be scanned is a two dimensional grid with each dimension increasing in size an amount equal to the uncertainty. This increased scan area means that the amount of time required for acquiring increases. This increase time to acquire can cause problems for some communications systems since the time devoted to scanning and acquiring is time away from the communications function. This can typically be tolerated if acquisition is only required once in a satellites service life, such as in a geosychronous system where the satellites move together and acquisition is typically only required at the beginning of the mission. However, in a system where acquisition is required more often, such as in a non-geosychronous communications system where satellites are moving with respect to each other and move into an out of each other's view as often as once every 1/2 hour, a long acquisition time can be tolerated. In such a system, each time a satellite moves out of and then back into the view of the acquiring satellite, reacquisition must take place. The scanning process requires several minutes of missed communications for each and every reacquisition which is unacceptable for most communications systems. Since continuous or virtually continuous communications are desirable for many system, it is important to reduce the amount of time needed for acquisition in a lasercom system.
What is need therefore is an apparatus and a method which reduces the field-of-view which must be scanned to acquire an opposing terminal and does not require a wide field-of-view sensor.